The Truce at Bakura
Starting just one day after the Battle of Endor, a planet on the edge of Imperial space is the first step on the path of conquest for a new alien race. The only ones who receive word about it are the Rebel Alliance, intercepting a message intended for the Emperor pleading for help. The two sides have to put their conflict on hold and stand together in order to face this new threat.
But, after the book has concluded and this one planet has been defended, the entire invasion is quietly dropped and never seen again until about twenty five years later for one book in the New Jedi Order. Not that I thought the Ssi-ruuk were especially compelling, but if you're going to base a story around the beginnings of a whole new conflict, have a plan to see it through. Introducing things with no idea where it's going is how you get Disney canon.